The present invention relates to an electromagnetic clutch mounted on a driven device such as a compressor for an automobile air conditioning system and, more particularly, to an electromagnetic clutch in which a temperature fuse fused by slip heat generated by the trouble of the driven device is disposed in a yoke.
An electromagnetic clutch is disposed on a power transmission line between a driving device and a driven device, and is used as a switch to drive or stop the driven device. In order to prevent an adverse influence on peripheral devices when the driven device breaks down, in a conventional electromagnetic clutch, a temperature fuse, which is fused by slip heat generated by slip rotation between the friction surface of a rotor coupled to the driving device and the friction surface of an armature coupled to the driven device, is connected in series to an exciting coil in a yoke.
In such an electromagnetic clutch, as proposed in Japanese Patent Laid-Open No. 56-138529 (reference 1) and U.S. Pat. No. 5,687,823 (reference 2), the temperature fuse is disposed in the opening portion of the yoke opposing a side surface of the rotor which is opposite to its friction surface, thereby improving the response performance of the temperature fuse.
In the electromagnetic clutch proposed in reference 1, a coil bobbin formed with a coil leading groove and a temperature fuse holding portion is provided to one flange portion of a winding frame portion which opens in the radial direction. When winding an exciting coil on this coil bobbin, an intermediate winding portion is led out from the coil leading groove. The led exciting coil is cut to form a pair of connecting portions, and the connecting portions are connected to the lead wires of the temperature fuse through bonding terminals. A case for the temperature fuse is fixed as it is fitted in the holding portion. The initial winding end portion and terminal winding end portion of the exciting coil are connected to a positive lead wire and a negative lead wire, respectively.
In a coil assembly having the above structure, the positive and negative lead wires are led out from an insulating bush, inserted in the annular grooves of the yoke, and fixed with a resin (to be referred to as a cast resin hereinafter) cast into the annular grooves.
In the electromagnetic clutch proposed in reference 2, the initial winding end portion of the annular exciting coil is connected to a negative lead wire, and the winding end terminal portion constituting one connecting portion is connected to one lead wire of the temperature fuse. The other lead wire of the temperature fuse is connected to one end portion of an extension line prepared to extend the exciting coil, and a positive lead wire is connected to the other end portion of the extension line. A case for the temperature fuse is fitted in a fuse holding member applied on the exciting coil.
In a coil assembly having the above structure, the positive and negative lead wires are led out from an insulating bush, inserted in the annular grooves of the yoke, and fixed with a resin cast into the annular grooves, in the same manner as in the coil assembly proposed in reference 1. The temperature fuse is assembled to be close to the outer circumferential wall of the inner cylindrical portion of the yoke.
The electromagnetic clutch proposed in references 1 and 2 is mounted on the housing of a compressor for an automobile air conditioning system serving as a driven device. In references 1 and 2, the yoke is fixed to the side surface of the housing, and the rotor inserted with the yoke is rotatably supported by a support, projecting from the side surface of the housing, through a bearing. An armature is supported on the shaft end of a rotary shaft projecting from the support, to be movable only in the axial direction.
In the electromagnetic clutch having this arrangement, when the friction surface of the armature is magnetically attracted by the friction surface of the rotor rotated by driving an automobile engine with the magnetic flux of the energized exciting coil, the compressor is driven. When energization to the exciting coil is disconnected, the magnetic flux disappears, and the armature is separated from the rotor, thereby stopping the driving operation of the compressor.
While the armature is magnetically attracted by the rotor, the rotary shaft of the compressor is locked, so that the two friction surfaces perform slip rotation. Accordingly, the environmental temperature abnormally increases sharply to fuse the temperature fuse. More specifically, the safety switch comprising the temperature fuse provided midway along the winding portion of the exciting coil is turned off to disable energization to the excitation coil. Accordingly, the exciting magnetic flux disappears to separate the armature from the rotor. As a result, a situation in which the belt wound on the rotor is disconnected to disable the driving operation of the other auxiliary machine of the automobile can be prevented.
In the conventional electromagnetic clutch, a pair of connecting portions of the exciting coil are integrally connected to the lead wires of the temperature fuse with the bonding terminals. The case for the temperature fuse is fixed by the holding portion of the coil bobbin or held by the holding member fitted on the exciting coil. This prevents the connecting portions from being broken by the temperature fuse or bonding terminals upon vibration of the automobile engine or compressor. However, since the connecting portions of the exciting coil and the lead wires of the temperature fuse must be connected with the bonding terminals, the assembly operation becomes cumbersome, and an improvement in productivity is difficult to achieve.
In the electromagnetic clutch of reference 2, due to the thermal expansion of the cast resin, a tensile load acts on the lead wires of the temperature fuse. Hence, to cover the lead wires with protection tubes or to form a bent portions (flex portions) on the lead wires is proposed. Since a resin that can be molded at a comparatively low temperature is used as the cast resin, according to the conventional explanation, the temperature fuse will not be fused during molding.
Along with an improvement in productivity and decrease in cost of an electromagnetic clutch for a compressor, this resin molding operation tends to be abolished. Accordingly, another holding means for holding the exciting coil and temperature fuse must be examined.
To cover the surface of the temperature fuse with a thin film and to embed the temperature fuse in a cast resin is also proposed. However, since variations in covering amount of the resin appear as variations in response performance of the temperature fuse, the process control of the quality of the casting operation need be performed with much labor.